I have been a vegetarian for about 15 years and at 6' 4" use to weigh 227 Lbs. A year ago my doctor told me that I was pre-diabetic, my A1c was 5.7 several times in the past but only once in a while and never above 5.7 now it was 5.7 two times in a row and that is considered the minimum threshold for pre-diabetic. My dad was type 2 diabetic in his later years, and so was his dad, and it caused health problems for both of them. So I decided to read books about pre-diabetis and got several out at the local library and read each one. There was one book that stood out as the best. It is "The prediabetes diet plan how to reverse prediabetes and prevent diabetes through healthy eating and exercise" by Hillary Wright
That book explains that when you are pre-diabetic your pancreas is being overwhelmed because the cells in your body are not processing insulin efficiently enough to use the insulin properly to convert the sugars in your blood into the sugars that your cells can use as food. The result is your pancreas sees excess amount of sugar in your blood and produces even more insulin. If this is allowed to go on long enough the overworking of your pancreas damages it and it looses the ability to produce insulin and then you are a diabetic. Once this happens it is not reversible. But if it has not progressed to that stage you can prevent it by limiting the amount of carbohydrates you consume. The book basically says to limit carbs to 60 or less for your three meals breakfast, lunch, and supper, and to less than 30 for two snacks. It goes on to say that if you are a tall person who is big that limiting the meals to 75 carbs or less in enough.
I took the threat of becoming a diabetic seriously, and began limiting carbs to 60 or less per meal, and 30 or less per snack. And soon after doing this discovered that I was loosing weight. I always try to walk for exercise so I already had the exercise part of the book taken care of. It took about 6 months and I went from 227 Lbs to 177 Lbs. My A1c went from 5.7 to 5.5 that does not sound like a lot but 5.5 is in the good range. It was just recently checked again and now it is 5.4 I have put on 3 pounds with the limits of not being able to go in stores to purchase all the foods I normally would eat, due to C-19 causing me to only use drive-by pick-up service and the selection of items limited what the stores post online. But even at 180 Lbs and 6' 4" I am still a healthy weight for my size and my BMI is well below 25 which is another thing the book tells you to achieve to prevent diabetes.
One of the most important things I found in any kind of healthy eating is to get a scale that can measure food weight in grams and look at the labels of each food and figure out how many carbs the weight of that food you are eating adds up to.
Egg white is a zero carb food. I have 100 grams of egg white cooked up almost every morning with other items and keep the carb total for breakfast to less than 60.
Lunch is a hard-boiled egg and other items all keeping the carb total to less than 30, though according to the book I could go higher.
A low carb mid day snack such as an apple, or kind bar, or RXbar.
A supper of less than 60 carbs. Usually 1/2 a can of Amy's black bean chili is part of that meal. While Amy's is more expensive, at only using 1/2 a can per meal is is not that expensive and it taste great and is pre-cooked. I measure out 200 grams per 1/2 can use, and microwave it in a bowl with a dish over it so it does not splatter.
And a late day snack of less than 30 carbs.
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The above limits will cause you to loose weight.
One thing I did was talk my doctor into writing a perscription for the Freestyle Libre continuous blood glucose monitor and I saw the rise and fall of my blood sugar level after eating, and how eating too many carbs causes it to rise too fast and too high.
One of the things the book talked about is that some foods release their sugars fast, and some release them slow. You want to eat the ones that release them slow, and limit the amount of the ones that release them fast. I could see these differences with different foods with the Freestyle Libre. I had to pay for the reader and sensor myself and only bought one sensor which worked for 2 weeks. But during those 2 weeks I learned the importance of spacing what you eat apart by at least 2 hours so your body has time to digest the sugar and keeps the peaks low, instead of eating too much carbs too close together and having your blood sugar peak too high.
Another thing the book talked about is the importance of eating foods with fiber, and if the food has more than a certain amount of fiber in it you can subtract half the amount of fiber from the carbs because the fiber will stop that amount of carbs from being absorbed into your system.
Now after loosing the weight, I sometimes slightly exceed the 60 or 30 carb limits, but even when I do, I do not exceed that by much. And my A1c has still improved.
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The above is my experience and what works for me. The book I recommended is the best one out of several that I read.
Good luck in finding what works for you.